1. Field of the Invention
The invention relates to external signal supply unit and method that supply an external signal to an on-vehicle unit. The invention also relates to a vehicle state acquisition system.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. JP-A-2000-233615 discloses a vehicle state acquisition system having a wheel-side unit and a vehicle-body-side unit that acquires a state of a wheel on the basis of information supplied from the wheel-side unit. In the case where identifier information on the wheel-side unit is registered in the vehicle-body-side unit in this vehicle state acquisition system, some pieces of identifier information that have been received within a set period since the setting of a registration mode are registered in descending order of reception frequency. The number of the pieces of information is equal to the number of wheels. The identifier information transmitted from the wheel-side unit installed in the vehicle is received more frequently than the identifier information transmitted from a wheel-side unit in another vehicle. Therefore, this vehicle state acquisition system makes it possible to avoid erroneous registration of the identifier information transmitted from a wheel-side unit in another vehicle.
If identifier information is registered or inspected in a generally employed vehicle state acquisition system, an external signal supply unit supplies a wheel-side unit with a transmission command signal commanding transmission of identifier information as an external signal. In accordance therewith, the wheel-side unit transmits identifier information. This identifier information is registered in a vehicle-body-side unit or utilized for inspection.
In this case, if a transmission command signal is supplied from an external signal supply unit to wheel-side units, for example, in each of adjacent vehicles, identifier information is transmitted from each of the wheel-side units. As a result of radio interference, a vehicle-body-side unit may not receive the identifier information normally. In this case, the identifier information may not be registered or normally utilized for inspection in the vehicle-body-side unit. Also, radio collision may occur among a plurality of external signals. The vehicle-body-side unit cannot receive an external signal in a good condition, and no identifier information is transmitted therefrom. In this case as well, no identifier information is registered or the implementation of an inspection is impossible in the vehicle-body-side unit. Further, the wheel-side unit to which an external signal ought to have been supplied from the external signal supply unit may not actually receive the external signal. No identifier information is transmitted from the wheel-side unit, and the identifier information on the wheel-side unit is not registered or the implementation of an inspection is impossible in the vehicle-body-side unit. The operator cannot find out, for example, that the identifier information on the wheel-side unit has not been registered. For example, if a piece of identifier information that ought to be registered has not been registered, a correction such as addition of another piece of identifier information or the like is required. The operator makes this correction.